New Hot-Filament Ionization Gauge with Low Residual Current

The design and performance of a new type of hot-filament ionization gauge is described. The ions are extracted from the ionization region, focused through an aperture in a shield electrode, and then collected on a short, fine wire. The sensitivity factor of this gauge is about 13 Torr−1 for nitrogen. The ratio of the soft x-ray flux reaching the collector of the new gauge to that of a Bayard-Alpert gauge is calculated, and the x-ray limit of the new gauge estimated to be 3×10−13 Torr. Measurements at low pressures indicate that the x-ray limit cannot be greater than 7×10−13 Torr and may be considerably less. False pressure indications, caused by the electronic desorption of positive ions from chemisorbed gases on the grid (in particular, oxygen), are at least 500 times smaller in the new gauge than in the Bayard–Alpert gauge.